Xenodiagnosis is used in certain diseases where it is difficult to recover the pathogen by conventional methods, an example being Chagas' disease where it actually is the method of choice. When arthropods blood feed they frequently ingest far more organisms than expected based on the volume of the blood meal. Xenodiagnosis takes advantage of this "concentrating effect" in that uninfected natural vectors of a pathogen are allowed to blood feed on patients to recover the organism. Xenodiagnosis can have practical value in certain laboratory aspects of Lyme disease research because the spirochete frequently is difficult to recover from an animal's body fluids and tissues, but the feasibility of this procedure never had been tried in nonhuman primates. A rhesus monkey was infected with the JD-1 strain of B. burgdorferi by the bite of infected nymphal stage ticks, then 2 days later uninfected nymphs were placed in capsules on the animal and given the opportunity to blood feed over a 5-day period. After a 21 day holding period, the 7 ticks that partially or fully engorged with blood were dissected and examined for spirochetes using the direct immunofluorescent antibody method (DFA). All of specimens were DFA positive. This opens the way for using xenodiagnosis in other aspects of our Lyme program, with particular value in vaccine studies.